1. Field of the Invention
The present invention relates to a process of producing titanium oxide fine particles excellent in hydrophobicity.
2. Description of the Prior Art
Titanium oxide fine particles are widely used as a white pigment or an additive, for example, in cosmetics, paints, inks, various synthetic resin molded items, synthetic resin films, and toners for electrophotography. Since these titanium oxide fine particles are required to have dispersibility in various resins, lubricants, and the like, it is needed that the hydrophobicity is good.
Hitherto, for the treatment of rendering titanium oxide fine particles hydrophobic, a silicone oil is often used. As the method of treating titanium oxide fine particles to render them hydrophobic, for example, a method wherein titanium oxide fine particles are coated directly with various alkylpolysiloxanes or are coated with an emulsion of various alkylpolysiloxanes or a solution of various alkylpolysiloxanes in a solvent followed by drying and then baking (Japanese Patent Publication (kokoku) No. 49-1769 ( 1974 ) and Japanese Pre-examination Patent Publication (kokai) No. 56-16404 (1981)) , a method wherein titanium oxide fine particles are coated directly with a hydrosilyl group-containing siloxane or are coated with a solution of a hydrosilyl group-containing siloxane in a solvent followed by drying and then baking (Japanese Pre-examination Patent Publication (kokai) No. 57-200306 (1982)), a method wherein titanium oxide powder brought in contact with a vapor of a hydrosilyl group-containing cyclic siloxane so that a silicone polymer is supported on the powder surface (Japanese Pre-examination Patent Publication (kokai) No. 3-163172 (1991)), and a method wherein titanium oxide fine particles are directly coated with an alkoxysilane or are coated with a solution of an alkoxysilane in a solvent and then the coated titanium oxide fine particles are treated with water or steam to effect the hydrolysis condensation to form a film of a condensation product (Japanese Preexamination Patent Publication (kokai) No. 57-200306 (1982) are known.
However, any of the above methods is difficult to render titanium oxide fine particles hydrophobic satisfactorily because the amount of the silicone for the surface treatment is small and the chemical adhesion of the surface treatment film is weak.
Therefore, to give satisfactory hydrophobicity, a method wherein a large amount of a silicone oil is blended with titanium oxide powder is suggested (Japanese Preexamination Patent Publication (kokai) No. 54-14528 (1979), but, in this method, although the hydrophobicity becomes satisfactory, the obtained surface-treated powder particles agglomerate due to the large amount of the silicone oil, resulting in a state which cannot be called a powdery state.